Migrating to v17.0.0

CsvHelper 17 checks the whole header before it reports a problem. HeaderValidated is called once, with every header that wasn't found, instead of once for each member, and the default throws one HeaderValidationException that lists them all, instead of one for the first. Code that sets HeaderValidated, reads the properties of HeaderValidationException, or overrides CsvReader.ValidateHeader needs changes.

Header validation

HeaderValidated takes an array of InvalidHeader, each with the Names a member is mapped to and the Index of the name, and the reading context:

var config = new CsvConfiguration(CultureInfo.InvariantCulture)
{
    HeaderValidated = (isValid, headerNames, headerNameIndex, context) =>
    {
        if (!isValid)
        {
            Console.WriteLine($"Missing header: {string.Join(" or ", headerNames)}");
        }
    },
};

var config = new CsvConfiguration(CultureInfo.InvariantCulture)
{
    HeaderValidated = (invalidHeaders, context) =>
    {
        foreach (var invalidHeader in invalidHeaders)
        {
            Console.WriteLine($"Missing header: {string.Join(" or ", invalidHeader.Names)}");
        }
    },
};

• It is called when every header is found too, with an empty array. A function that throws or logs on every call needs to check the array first.
• Overriding ValidateHeader(ClassMap map): it is ValidateHeader(ClassMap map, List<InvalidHeader> invalidHeaders), and adds the headers it doesn't find to the list. ValidateHeader(Type) calls HeaderValidated once the whole map is checked.

HeaderValidationException

The exception describes every missing header, with a line for each in its message.